In the graphics art industry continuous paper webs are put through many finishing operations. Paper webs may be perforated, printed, slit into individual documents, folded or glued to produce a variety of products. Machinery to perform several operations may be combined as units of a continuous finishing line. When this is done, control schemes are utilized to adjust the speeds of the various individual processing units so that a common throughput rate is achieved. Dancer control systems are a known means for adjusting the speeds of individual units to a common throughput rate. A dancer system utilizes a dancer roller in a loop of running web. The dancer roller is mounted on an arm and moves up or do the amount of web in the loop decreases or increases. The movement of the dancer roller actuates a transducer device, and thereby continously adjusts the throughput rate of an operating unit to maintain the position of the dancer roller within a preselected range of travel.
Certain operations in high speed paper handling have large instant-to-instant speed changes, however. Existing and proposed printers have that feature. Laser printers, which are at present capable of printing at average web throughput rates in the order of 150 feet per minute, not only stop periodically, but even reverse the direction of web travel during their printing cycles. The reversal, which is believed to occur during cleaning of the printer's selenium drum, is followed by a very rapid acceleration of the web to its forward operating speed. This type of intermittent operation at high rates tends to result in uneven and high web tensions leading to web breakage, and consequent shutdown of the finishing line.
It is important for proper laser printer operation that the web be under tension which is both low and uniform through the laser printer. Other processing units which may be included with a printer in a finishing line, however, do not match the intermittent operation of the printer and, therefore, tend to produce uneven and high tension at the printer. For example, web leaving a printer may be rewound onto a roll. Rewinding, even under dancer control, does not allow for the printer's reversal, leading to web breakage when the relatively high tension printer reverses.